1. Field of the Invention
The present invention relates to a polyphthalatecarbonate resin and, more particularly, relates to a polyphthalatecarbonate resin which exhibits a low degree of yellowness and exhibits a high level of resistance to yellowing and hazing caused by ultraviolet degradation.
2. Description of Related Art
Aromatic polycarbonate resins have found utility in glazing applications. For example, these resins have been made into sheets and films which have been used as windows and the like. Aromatic polycarbonate resins, however, suffer from degradation upon prolonged exposure to ultraviolet radiation resulting in the resins becoming increasingly yellowed and hazy over time. Prior attempts to reduce this yellowing and hazing have involved either coating the sheets and films with ultraviolet radiation absorbing compounds such as benzophenones or blending such compounds in with the polycarbonate resins. These techniques have, however, experienced various shortcomings, including the expensiveness of the absorbers, the equipment requirements associated with coating processes, and the use of solvents which must be evaporated off during coating of the sheet.
Polyarylate resins also have found some utility in glazing applications, however, polyarylates are well known to have severe yellowing problems when exposed to ultraviolet light due to a photo-Fries rearrangement occurring at the ester bond linkage to an aryl group (See, for example, U.S. Pat. No. 4,534,104 and Adv. Photochem., 8, 109-159 (1971)). This ultraviolet light promoted reaction results in a rearrangement of the type illustrated below: ##STR1##
This type of degradation leaves a yellow colored layer within the exposed surface of the polyarylate resin as a product of the photo-Fries rearrangement. Additionally, polyarylate resins typically have relatively high initial levels of yellowness which make them less attractive than aromatic polycarbonate resins in glazing applications.
Accordingly, one object of the present invention is to provide a transparent thermoplastic resin which exhibits low levels of initial yellowness and high degrees of resistance to yellowing and hazing upon prolonged exposure of ultraviolet radiation thereto.
Another object of the present invention is to provide a transparent copolycarbonate resin which exhibits high degrees of resistance to yellowing and hazing from ultraviolet radiation without the need for the use of relatively expensive and, sometimes, toxic ultraviolet radiation absorbing additives.